Ganymede Fact Sheet
What Is Known About Ganymede SO FAR
Ganymede is the largest satellite in the solar system with a diameter of 5,268 km (3270 miles). It is larger than Mercury and Pluto, and three-quarters the size of Mars. If Ganymede orbited the Sun instead of orbiting Jupiter, it would easily be classified as a planet.
Since Ganymede has a low density of 1.94 grams/cubic centimeter (water's density = 1.00), it was originally estimated that the satellite is half water ice with a rocky core extending to half of the satellite's radius. However, the first two flybys by Galileo had detected a magnetic field around Ganymede, which strongly indicates that the satellite has metallic core about 250 to 800 miles in. The mantle is composed of ice and silicates and a crust which is probably a thick layer of water ice.
Voyager images of Ganymede have shown that the satellite has a complex geological history. Ganymede's surface is a mixture of two types of terrain. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain which forms intricate patterns across Ganymede. The term "sulcus," meaning a groove or burrow, is often used to describe the grooved features. This grooved terrain is probably formed by tensional faulting or the release of water from beneath the surface. Groove ridges as high as 700 meters (2000 feet) have been observed in the Voyager imagery and the grooves run for thousands of kilometers across Ganymede's surface. The grooves have relatively few craters and probably developed at the expense of the darker crust. The dark regions on Ganymede are old and rough, and the dark, cratered terrain is believed to be the original crust of the satellite. Lighter regions are young and smooth (unlike the Moon). The largest area on Ganymede is called Galileo Regio, and is one of the areas targeted by the Galileo spacecraft.
The large craters on Ganymede have almost no vertical relief and are quite flat. They lack central depressions common to craters often seen on the rocky surface of the Moon. This is probably due to slow and gradual adjustment to the soft icy surface. These large "phantom craters" are called palimpsests, a term originally applied to reused ancient writing materials on which older writing was still visible underneath newer writing. Palimpsests range from 50 to 400 km in diameter. Both bright and dark rays of ejecta exist around Ganymede's craters - rays tend to be bright from craters in the grooved terrain and dark from the dark cratered terrain.
Recently, the Hubble Space Telescope detected a thin oxygen atmosphere on Ganymede.
Discovery: Jan 11, 1610 by Galileo Galilei Diameter (km): 5,268 Mass (kg): 1.48e23 kg Mass (Earth = 1) 0.0247 Surface Gravity (Earth = 1): 0.145 Mean Distance from Jupiter (km): 1,070,000 Mean Distance From Jupiter (Rj): 15.1 Mean Distance from Sun (AU): 5.203 Orbital period (days): 7.154553 Rotational period (days): 7.154553 Density (gm/cm?3) 1.94 Orbit Eccentricity: 0.002 Orbit Inclination (degrees): 0.183 Orbit Speed (km/sec): 10.9 Escape velocity (km/sec): 2.74 Visual Albedo: 0.43 Subsolar Temperature (K): 156 Equatorial Subsurface Temperature (K): 117 Surface Composition: Dirty Ice
The Discovery of the Galilean Satellites
Find out how Galileo Galilei discovered the moons around Jupiter.